The basic building blocks of CSS are rule sets. Each rule set is comprised of a selector and a declaration block. Each declaration block has zero or more declarations, or styles. Here is a basic rule set:
p {
background-color:#ffcc00;
font-size:120%
}

One important practice when creating Cascading Style Sheets (CSS) is to include comprehensive comments in the files. This will help to keep things organized and well documented, which is priceless when it comes to maintaining a site for several years, and avoiding future frustration!
This is the basic structure:
/* your comment here */